1. Field of the Invention
The present invention relates to a gearshift control apparatus for an automatic transmission wherein clutch-to-clutch gearshifting is effected.
2. Description of the Prior Art
In clutch-to-clutch gearshifting, the changeover timing of both clutches to be released and to be engaged is important. The official gazette of Japanese Patent Application Publication No. 51984/1995 discloses an example wherein, in case of a power-ON upshift, the changeover timing of both the clutches is established by subjecting the hydraulic pressure of the release side clutch to a feedback control so that the r.p.m. of an engine may become target r.p.m. on their blowing-up side (the target r.p.m. being that value of the engine r.p.m. which is somewhat greater than the synchronous r.p.m. of the lower-speed-stage side before lowering), and also by gradually raising the torque capacity of the engagement side clutch. That is, in this example, the changeover timing of the clutches is established by feedback-controlling the hydraulic pressure so that the release side clutch may continue a slight slip state.
The example is based on technical ideas as mentioned below. (1) The hydraulic pressure of the release side clutch is kept on the blowing-up side of the engine r.p.m., whereby the "tie-up", namely, the braking caused by engaging of both the engagement side clutch and the release side clutch can be prevented from occurring. (2) Since the hydraulic pressure is fed back so as to maintain the slight slip state, the excessive blowing-up of the engine r.p.m. can be prevented by the feedback. (3) When the engagement side clutch comes to have the torque capacity, the release side clutch has its r.p.m. lowered toward the synchronous r.p.m. and is naturally released owing to the feedback for keeping the "value greater" than the synchronous r.p.m., so that the changeover is smoothened.
The above control, however, involves the problem that the operation of "feedback-controlling the hydraulic pressure of the release side clutch for the target r.p.m. on the blowing-up side of the engine r.p.m." is actually very difficult.
More specifically, the clutch on the release side is held in perfect engagement (under a high hydraulic pressure) immediately after the issue of a gearshift command. When the hydraulic pressure is lowered little by little, the clutch begins to slip critically (or suddenly) at a certain point.
Therefore, when the rate of lowering the hydraulic pressure is high, the clutch slips very greatly since the beginning of the slip, to incur the phenomenon that the feedback control cannot follow up the great slip. When the feedback gain of the feedback control is heightened in order to cope with the phenomenon, hunting ascribable to an excessive correction arises easily. Since the feedback gain is greatly affected by manufactural discrepancies and secular changes, it is very difficult to set the feedback gain at an appropriate value on each occasion.
On the other hand, when the rate of lowering the hydraulic pressure is low, such drawbacks can be eliminated to some extent. In this case, however, a very long time is expended in reaching the critical point from the state of the high engagement pressure. This results in the problem that a gearshifting time period becomes extraordinarily long, so the gearshift stage intended by the driver of a motor vehicle is achieved slow accordingly.